One of the great political biographies of the last 20 years is Roy Jenkins' Gladstone. What makes it so specially enjoyable is the sense throughout that Jenkins is conducting a conversation with his subject on the nature of political power.

The shortlist of four, drawn up by Labour MPs, was Labour's first MP Keir Hardie, its first prime minister of a majority government, Clement Attlee, its greatest woman politician so far, Barbara Castle and the founder of the NHS Nye Bevan. Bevan's champion was Ed Balls, Gordon Brown's closest ally.

During conversations when I was writing her biography, Castle always insisted that the first question any politician should ask of another was, "what did he mean by that?" So what were we to read into the decision of the leading Brownite Ed Balls to argue the case for the legendary Labour schismatic, Aneurin Bevan?

Bevan has long stood for everything that is most inspiring about Labour. A South Wales miner who left school at 13, whose first taste of political power was organising during the 1926 General strike and the miners' strike that lasted long after it, Nye Bevan was a radical visionary in the 1930s, a courageous thorn in Churchill's side in the 1940s and of course ultimately father of the NHS.

But he was also the politician, in the late 1940s when, just as in the late 1920s and again in the 1960s, the Labour government fell apart under the pressures of extreme economic events, who was at the head of a band of what the leadership saw as schismatic die-hards.

They refused to accept that rearmament was more important than the NHS, and later consolidated round opposition to the Atlanticism of the Labour leadership in what was partly a very personal battle between the great romantic son of the valleys and Hugh Gaitskell, the man he once branded a "dessicated calculating machine".

Balls had to defend his hero against David Blunkett (supporting Attlee), who branded him a quitter, arguing that the odd moment of misplaced passion should not discredit him:

But at the very end of a career truncated by early death, Bevan recanted: he accepted nuclear weapons and prepared for a role as foreign secretary. And what attracted Balls, we learned, was not Bevan's oratory or his passion but his understanding that without power, there was no purpose in politics. There have to be limits to radicalism. It was Bevan who said the "religion of socialism is the language of priorities".

It is not hard to imagine that where Balls said Bevan, we might also discern a lightly disguised reference to Brown. And when he asserted that Bevan had never been a Bevanite, and in fact had been embarrassed by the misplaced passion of his supporters, one suddenly understood.

There had never been Brown-Blair tribal war, let alone one that lasted even longer than the Bevan-Gaitskell split, it was all got up by the prime minister's sidekicks, presumably not including Ed Balls. In an early aside Balls himself reinforced the appeal to unity with the unexpected opinion that Tony Blair should have been on the list.

It was a great meeting: at last there was a safe space for sentiment. There were people there who'd met Clement Attlee; one person had shared a platform with Ramsay MacDonald, although only in his mother's arms, another had been the shop steward of the Fords' sewing machinists whose strike led to the equal pay legislation. After years when new Labour seemed in denial about its history, there was a spirit of celebration about the opportunity to argue over what really mattered to the party.

It was of course a travesty that Barbara Castle didn't win. Listen here as Fiona Mactaggart, her champion, explains how she represented both pragmatism and inspiration:

But tellingly, it was Kenneth Morgan's improbable picture of Keir Hardie handling the 21st century collapse of capitalism that wowed today's audience:

Scratch any Labour activist, and there is always a powerful desire for principle. Which is why no leader can ever quite live up to them.